|Suspended Sediment Concentration (Surface Runoff)|
|Suspended Sediment Concentration in Runoff|
|Achieve a 90 percentile concentration value for suspended sediment of 250 milligrams/liter.|
|:||The proportion of individual samples that exceed 250 milligrams/liter.|
No subcategories for this indicator.
Approach Monitoring is guided by the RSWMP Framework and Implementation Guidance document. During water year 2014 five catchments were monitored for continuous flow and turbidity and sampled for water quality at eleven monitoring stations: the outfalls of the five selected catchments, and the inflows to and outflows from selected BMPs located in three of those catchments. Three additional catchment outfalls were monitored in water year 2015. The catchments were chosen because of their direct hydrologic connectivity to Lake Tahoe, diversity of urban land uses, range of sizes, and a reasonably equitable distribution among the participating jurisdictions. BMP effectiveness sites were selected because of their potential efficacy in treating storm water runoff characteristic of the Lake Tahoe Basin, and the broad interest in, and lack of conclusive data regarding the efficiency of the selected BMPs in reducing runoff volumes and pollutant loads.
Associated Programs data not provided.
Fine suspended sediment (fine sediment particles) are the primary driver of the decline in lake clarity (responsible for 55 to 60 percent of the decline) and urban upland areas are the dominate source of fine sediment particles in the Region (Lahontan and NDEP, 2010a).Human and Environmental Drivers
– Landscape modification (e.g. impervious cover such as roads or residential and commercial development or logging) influences the volume of runoff, erosion rates, and the ability of the watershed to retain sediment and nutrients. The concentration of sediment and nutrients in stormwater runoff is influenced by the type, magnitude, and location of landscape modifications. Concentration is further mediated by the extent to which practices to mitigate potential impacts are in place. A variety of natural factors also influence the concentration of sediment and nutrients in stormwater concentrations including climate, weather, landscape topography, and vegetation. The Lake Tahoe TMDL estimated that urban upland areas contributed 348 metric tons of fine sediment particles to the lake annually, 72 percent of the annual total (Lahontan and NDEP, 2010a).
Achieve a 90 percentile concentration value for suspended sediment of 250 mg/1.
Attain a 90th percentile value for suspended sediment concentration of 60 mg/L.
Insufficient data to determine status. The sampling design and the small fraction of the outflows points and storm events sampled preclude a status determination on the concentration of stormwater inflow points. Further discussion is included in the recommendations sections. Of the small fraction of outflow points samples in water years 2014 and 2015, 64 percent of sampled outflows were better than the standard of 250 milligrams/liter for total suspended solids, while 74 percent of sampled outflows were better than the standard of 250 milligrams/liter for fine sediment particles (Tahoe Resource Conservation District, 2015).
Insufficient data to determine trend. Stormwater monitoring under the IMP component of RSWMP began in 2014. The IMP of RSWMP is currently funded through 2019 by SNPLMA, and trend assessments are expected to be included in future evaluations.
Status: Moderate. There is moderate confidence in the data because it is collected using widely recognized, standardized national protocols (see monitoring approach) with quality assurance/quality control procedures. Only a small proportion of outflows are sampled and not all runoff events are sampled. Regional concentration estimates are not available at this time. Current sampling tests for total suspended solids and fine sediment particles, limiting the ability to report on suspended sediment standards therefore reducing the confidence to moderate.
Trend: Low. No trend assessment was performed because both the nature and limited duration of the data preclude trend assessment.
Overall: Low. Overall confidence takes the lower of the two determinations.
Urban growth control limits, best management practices (BMPs) to reduce nutrient and sediment discharge from disturbed soils, BMP retrofit regulations for developed properties, reducing private automobile use through improvements to public transit and alternative transportation modes (with the goal of reducing air pollution and the subsequent deposition of nitrogen and fine sediment), and ongoing allocation of water quality mitigation funds to support erosion control and storm water pollution control projects. Projects completed by EIP partners since between 2009 and 2015 have:
The Regional Plan requires the use of best management practices (BMPs) for new residential and commercial development, and BMP retrofit regulations for developed properties. For example, section 60.4.6.A.1 of TRPA Code requires properties be able to infiltrate the 20-year, one-hour storm into groundwater. The Regional Plan is also designed to limit growth and shift development from sensitive to less sensitive lands. All of these requirements contribute to reducing fine sediment and nutrient runoff from developed areas. The Regional Transportation Plan complements these by encouraging use of public transit and alternative transportation modes, and reducing reliance on private automobile. Water quality mitigation fees, collected on projects that create new cover, support erosion and storm water pollution control projects. Projects completed by EIP partners since between 2009 and 2015 have:
• Retrofitted 120.55 miles of road and decommissioned an additional 7.4 miles of road.
• Inspected 108.72 miles of unpaved non-urban roads and maintained 98.2 miles.
• Issued 18,076 BMP certificates to developed commercial, multifamily and single family residential properties.
• TRPA’s grant funded Stormwater Management Program (SMP) focuses compliance and maintenance verification activities on priority commercial and large multi-family residential properties in coordination with local jurisdictions. In 2015, the SMP notified 2,441 parcel owners with BMP Certificates issued more than five years ago that maintenance was due and re-issued 186 BMP Certificates following maintenance verification.
• Completed street sweeping on 24,644 miles of roads.
The TRPA Stormwater Management Program leads broad professional and public education including annual BMP trainings for contractors, local jurisdictions and real estate professionals, articles in “Tahoe In-Depth” mailed to all property owners, and public workshops and events to increase BMP awareness and promote proper design, installation and maintenance. Public outreach and educational campaigns (such as the “Take Care” campaign) highlight for residents and visitors what they can do to maintain a healthy environment including BMP completion. Between 2012 and 2015 the South Tahoe Environmental Education Coalition delivered 36 educational programs and reached nearly 30,000 individuals.
The Lake-Friendly Business Program highlights and encourages patrons to visit businesses that are doing their part to help protect Lake Tahoe by installing and maintaining their water quality BMPs. There are currently over fifty Lake-Friendly businesses in the Region.
The TMDL Management System Handbook guides the actions of agencies in the Region to reduce inputs of nutrients and sediments into Lake Tahoe (Lahontan and NDEP, 2014). As part of the TMDL implementation, each jurisdiction in the Region prepares a load reduction plan (pollutant load reduction plans in California and stormwater load reduction plans in Nevada) that detail the steps to achieve the specified load reductions. The Lake Tahoe TMDL estimated that a 50 percent reduction in nitrogen load from urban sources (8 percent of the total nitrogen load) would be required to achieve lake clarity standards (Lahontan and NDEP, 2010b).
The 2015 TMDL Findings and Recommendations memo identified wintertime traction abrasives as a primary source of ultra-fine sediment particles (less than 16 microns in stormwater runoff) (Larsen and Kuchnicki 2015a). Managers and heavy equipment operators in the Tahoe Region continue to adaptively manage wintertime traction application practices to reduce adverse environmental impacts while ensuring safe roads. In the 2015/2016 winter season this included treating roadways with brine solution prior to storm events, which prevents ice from developing on roads and can reduce prior dry salt applications by as much as 86 percent (Wigart and Ferry 2015b). El Dorado County, the California Department of Transportation and the City of South Lake Tahoe are utilizing new wintertime traction abrasives that contain 90 percent less ultra-fine particles compared to previously used materials and also break down less into fine fractions from vehicle traffic. This new abrasive is sourced from a native granite material rather than the previously imported non-native volcanic cinders (Wigart and Ferry 2015a).
Each year the actions of the TMDL implementation partners are summarized and evaluated in the TMDL Performance Report. The pollutant tracking system for urban stormwater was being refined during the reporting period. Future evaluations will use the estimated reductions in urban source pollutants to assess the effectiveness of programs and actions implemented to reduce pollutant load from urban sources (Larsen and Kuchnicki, 2015a).
TRPA infiltration requirements were designed to strike a balance between environmental benefit and cost. A 2011 synthesis of existing knowledge found diminishing returns from increasing storm retention capacity beyond the 20-year, one-hour storm. The synthesis found that doubling retention capacity required to handle the 20-year, one-hour storm would only increase annual retention by seven percent (2ndNature and NHC, 2011). TRPA Code Section 60.4.6.A.1 further requires a one-foot separation between seasonal high groundwater and the bottom of an infiltration system to protect groundwater resources.
The 2014-2015 Stormwater Monitoring Report analyzed the effectiveness of BMPs in the Region (Tahoe Resource Conservation District, 2015). At BMP’s where both inflow and outflow were monitored, reported results included:
• The Central Incline Village Phase II Project, which included three upstream infiltration basins, two small roadside infiltration pools, 450 linear feet of roadside infiltration channel and a Jellyfish treatment vault, effectively eliminated flows and pollutant loads reaching Lake Tahoe (Tahoe Resource Conservation District, 2015).
• 66 percent of inflow samples (42 of 64) and 83 percent (53 of 64) of outflow samples met the TRPA suspended sediment concentration threshold of 250 milligrams/liter. However, 18 of the 53 outflow samples that met the standard were dry samples (dry samples indicate that the BMP was effective in infiltrating the storm event).
• Average suspended sediment concentration was reduced by 46 percent between inflow and outflow. Average outflow concentrations includes values of zero milligrams/liter where there was no measured flow leaving the BMP.
An interim target cannot be set at this time, due to the duration of the data.Target Attainment Date
A target attainment date cannot be set at this time, due to the duration of the data.
Explore methods to extrapolate from limited field survey data to ambient concentrations at unmonitored locations. The PLRM, which is used for load estimation, uses an event mean concentration - i.e. a single value for a runoff event, and is thus not suitable for evaluation of the concentration based standards.Monitoring Approach
No changes recommendedModification of the Threshold Standard or Indicator
Standard revision should consider consistency with the pollutant load-based standards and management strategies implemented through the Lake Tahoe Total Maximum Daily Load (TMDL) program per water quality policy 1.5 of the TRPA Regional Plan.Attain or Maintain Threshold
The 2015 Findings & Program Recommendation Memo for the TMDL reported that no new findings relative to urban stormwater were reported in previous calendar year (Larsen and Kuchnicki, 2015b). Continue to implement the programs of the TMDL.
Map description not provided.
No figures available.
The 2011 Evaluation Report has not yet be uploaded or is unavailable.